For road racing, the 3.31 and M20 combination does make some sense. So if someone knew what they wanted, it could have been ordered that way. But most received the 3.73 or 4.10 axles.
I run an M21 behind a 500+ hp big block with a 3.31 rear end. It’s not a rocket off the line, so you have to baby it a little at take off. Once you get rolling, no problem - the torque takes over. Someday I may swap in an M20.